1. Field of the Invention
This invention relates generally to dilatometers and, more particularly, to improved test specimen support platens which can be used to transmit a deforming force to the specimen and, when the deforming force is absent, also can be used to transmit thermally induced changes in the linear dimensions of the specimen to the dilatometer sensor.
2. Description of the Prior Art
Dilatometers are analytical instruments that respond to the linear thermal expansion or contraction of solids. Typically, these instruments have a variable temperature furnace in which the test specimen rests between a flat surface on a stationary object and an opposing flat surface on a movable object, such as a ceramic rod, that extends outside the furnace. Temperature induced changes in the length of the specimen are transmitted through the rod to a mechanical, optical or electrical system for amplifying and measuring that change. These instruments can be used to make precise measurements of changes in length resulting from small temperature changes or to plot variations in the rate of linear expansion or contraction over a broad temperature range. Such measurements are invaluable in studying the compatibility under changing temperature conditions of different materials which are bonded together or are in close contact; e.g., metal to glass, enamel to substrates, thin film deposits in microcurcuits or metal or plastic fillings in natural teeth. Exemplary of the dilatometers that are commonly used for such applications are the instruments described in U.S. Pat. Nos. 3,680,357 and 3,898,836.
Dilatometers also can be used to detect and measure the precise temperature at which phase transitions occur in a specimen material. One such application is the study of the crystalline structure of steel and the effect on that structure of the heating and cooling rates to which it has been subjected. By simulating various steel mill operating capabilities, it is possible to determine the optimum heating or cooling rate for a specific alloy which will result in a product having desired properties or combinations of properties. A dilatometer designed specifically for this purpose is described in U.S. Pat. No. 3,805,589. In this instrument, linear dimensional changes of the specimen are transmitted through a ceramic push rod to the independently suspended core of a linear variable differential transformer that converts these dimensional changes to electric signals. When amplified and plotted against temperature and time, these signals clearly show phase changes. Variable rates and intensities of heating of the specimen are effected by controlling the current fed to an induction coil wound about the specimen and variable rates and intensities of cooling are effected by controlling the volume of a jet of quenching fluid, such as helium, that is directed either into a passageway through the specimen or on its outer surface. Although this quenching dilatometer accurately simulates steel mill production of cast products and is widely used to determine optimum heating and cooling rates, it does not fully reflect the effect of forging or rolling on transition temperatures or crystal structure.
In order to simulate these common steel mill deforming operations, conventional quenching dilatometers have been modified to provide the capability of crushing the specimen between the heating and quenching stages, as illustrated by the pioneer model deformation dilatometer described by Smith and Siebert, "Transformation Kinetics of Thermomechanically Worked Austenite by Deformation Dilatometry", Applications of Modern Metallographic Techniques, ASTM STP 480, American Society for Testing and Materials, 1970, pp. 131-151. Specimen deformation is accomplished with this instrument by holding the cylindrical specimen vertically between a flat smooth surface on a lower fixed corderite platen and a parallel opposed flat smooth surface on an upper movable corderite platen. After the specimen is heated, a deforming force is applied, via a hydraulic cylinder, to the movable platen normal to and in the direction of its flat smooth surface that is in contact with the specimen. After withdrawal of the deforming force, the specimen is quenched with a jet of helium and further dimensional changes are transmitted to the core of a linear variable differential transformer through a pushrod that rests on the movable platen.
While this early deformation dilatometer led to a much improved understanding of the effect of forging or rolling on the crystalline structure of steel, the data produced often was distorted by dimensional changes occurring in the corderite platens and the instrument was of limited use in identifying optimum heating and cooling cycles for steel mill forging or rolling operations.
In response to this recognized shortcoming, more sophisticated versions of the deformation dilatometer were developed which automatically compensate for thermally induced dimensional changes in the corderite platens by employing two pushrods, each of which is coupled to a different element (core or coil) of a linear variable differential transformer in which these elements are independently movable. One of these pushrods abuts the flat smooth surface of the movable platen adjacent the specimen and is responsive to the cumulative linear expansion or contraction of the specimen and the fixed platen. The second pushrod, which is coupled to the other independently movable element of the linear variable differential transformer, abuts a lip adjacent the specimen bearing surface of the fixed platen and, being responsive to only the linear expansion or contraction of the fixed platen, cancels the electrical output reflecting that dimensional change.
While the elimination of this platen error greatly improved the accuracy of the deformation dilatometer for many laboratory measurements, the continued frequent occurrence of erroneous results and the difficulty of identifying same has limited the widespread use of this instrument to establish optimum heating and cooling cycles for steel mill forging and rolling operations. Prior to this invention, the cause of these erratic results was not understood and could not be avoided.